In recent years, from the viewpoint of exhaustion of oil resources estimated in the near future and because of soaring crude oil prices, deep oil fields which have not been searched and oil fields, gas fields, and the like in severe corrosion environments so-called sour environments containing hydrogen sulfide and the like have been actively developed. In general, such oil fields and gas fields have very large depths and the atmospheres thereof are severe corrosion environments containing CO2, Cl−, and furthermore, H2S at high temperatures. Oil country tubular goods (OCTG) used in such environments are required to include materials having predetermined high strength and excellent corrosion resistance in combination.
In oil fields and gas fields in environments containing carbon dioxide CO2, chlorine ions Cl−, and the like, in many cases, 13% Cr martensitic stainless steel pipes have been employed as oil country tubular goods used for development drilling. In addition, recently, use of improved version 13% Cr martensitic stainless steel has been spread, where C is reduced and Ni, Mo, and the like are increased in the component system of 13% Cr martensitic stainless steel.
For example, Patent Literature 1 describes an improved version 13% Cr martensitic stainless steel (steel pipe), where the corrosion resistance of the 13% Cr martensitic stainless steel (steel pipe) is improved. The stainless steel (steel pipe) described in Patent Literature 1 is a martensitic stainless steel having excellent corrosion resistance and excellent sulfide stress corrosion cracking resistance, wherein in the composition of martensitic stainless steel containing 10% to 15% of Cr, C is limited to 0.005% to 0.05%, Ni: 4.0% or more and Cu: 0.5% to 3% are added in combination, 1.0% to 3.0% of Mo is further added, and Nieq is adjusted to −10 or more and the microstructure is composed of a tempered martensitic phase, a martensitic phase, and a retained austenitic phase, while a total fraction of tempered martensitic phase and martensitic phase is 60% to 90%. It is mentioned that the corrosion resistance and the sulfide stress corrosion cracking resistance are thereby improved in wet carbon dioxide environments and in wet hydrogen sulfide environments.
Meanwhile, Patent Literature 2 describes a martensitic stainless steel containing, on a percent by mass basis, C: 0.01% to 0.1%, Si: 0.05% to 1.0%, Mn: 0.05% to 1.5%, P: 0.03% or less, S: 0.01% or less, Cr: 9% to 15%, Ni: 0.1% to 4.5%, Al: 0.0005% to 0.05%, and N: 0.1% or less, wherein C+63N satisfies 0.029 to 0.072 and the proof stress is 758 to 965 MPa in a state as cooled or as normalized after hot working. Also, in the technology described in Patent Literature 2, at least one selected from Mo: 0.05% to 3% and Cu: 0.05% to 5.0% and/or at least one selected from Ti: 0.005% to 0.5%, V: 0.005% to 0.5%, and Nb: 0.005% to 0.5% may be further contained. It is mentioned that the proof stress can be thereby specified to be within the range of 758 to 965 MPa and a martensitic stainless steel (steel pipe) with high reliability can be produced.
Also, Patent Literature 3 describes a martensitic stainless steel containing, on a percent by mass basis, C: 0.01% to 0.10%, Si: 0.05% to 1.0%, Mn: 0.05% to 1.5%, P: 0.03% or less, S: 0.01% or less, Cr: 9% to 15%, Ni: 0.1% to 4.5%, Cu: 0.05% to 5%, Mo: 0% to 5%, Al: 0.05% or less, and N: 0.1% or less, wherein Mo+Cu/4 satisfies 0.2% to 5%, the hardness HRC is 30 to 45, and the amount of carbides at primary austenite grain boundaries in the steel is 0.5 percent by volume or less. In the technology described in Patent Literature 3, at least one selected from Ti: 0.005% to 0.5%, V: 0.005% to 0.5%, and Nb: 0.005% to 0.5% may be further contained. It is mentioned that any corrosion resistance of the sulfide stress corrosion cracking resistance, the wear resistance and corrosion resistance, and the localized corrosion resistance can be thereby satisfied even in the use in an environment containing carbon dioxide and a very small amount of hydrogen sulfide.
Also, Patent Literature 4 describes a stainless steel pipe for oil well use, having a steel composition containing, on a percent by mass basis, C: 0.05% or less, Si: 0.50% or less, Mn: 0.20% to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0% to 18.0%, Ni: 5.0% to 8.0%, Mo: 1.5% to 3.5%, Cu: 0.5% to 3.5%, Al: 0.05% or less, V: 0.20% or less, N: 0.01% to 0.15%, and O: 0.006% or less, wherein Cr, Ni, Mo, Cu, and C satisfy a specific relationship and, furthermore, Cr, Mo, Si, C, Mn, Ni, Cu, and N satisfy a specific relationship.
In the technology described in Patent Literature 4, at least one selected from Nb: 0.20% or less and Ti: 0.30% or less may be further contained. It is mentioned that a martensitic stainless steel pipe having sufficient corrosion resistance even in severe corrosion environments containing CO2 and Cl− at high temperatures can be produced.